The invention relates to a circuit arrangement for inverters with three stage commutation, particularly suitable for inverters of large output with parallel connection of main thyristors.
Self commutated inverters are known wherein the commutation process proceeds in two stages, so-called inverters with two stage commutation. The commutation capacitors of these inverters must in addition to achieving the proper blocking of the thyristor, also store an accumulation of power from the induction load, i.e. in asynchronous motors, the power of the magnetic stray field. This leads to a large capacity of these capacitors and to substantial difficulties at limit conditions, for instance to a limited frequency range. These problems are solved by application of a three stage commutation, where the functions of the commutation circuit are separated to the proper disconnection of the thyristors and the accumulation of power. It is possible to propose, on the base of a three stage commutation, a number of different inverter circuit arrangements such as, the arrangement according to the Czechoslovak certificate of authors of an invention No. 169.899. The use of three stage commutation however does not solve all problems of inverter circuits, for instance overloading, a reliable commutation with valves connected in parallel, with separating reactors, etc. The inverter with three stage commutation according to the above mentioned certificate No. 169.899 is not suitable for large outputs, particularly not for a connection in parallel of main thyristors and separating reactors. This is due to the circumstance that, in the case of an increase of commutation currents, the inductances in the commutation circuits are lowered and are comparable to the inductances of the separating reactors, possibly including the selfinductance of connecting bars and supply conductors. Together with requirements for overloading where requirements on commutation ability are furthermore increased and inductances in the commutation circuits are therefore decreasing, this circuit arrangement is not suitable.